The present invention relates to extraction towers for sugar beets cossettes or crushed sugar cane to be leached in a counter-current fashion. To this end the conveying means in the tower comprise a central shaft rotatably supported coaxially in the tower. The shaft carries a plurality of conveying elements in the form of wing or worm screw flights. Additional conveying elements are secured to the inner surface of the housing forming the tower. These additional conveying elements comprise guide plates or baffle plates pivotally held in bearings for tilting about their longitudinal axis, said bearings being secured to the wall of the tower. The guide or baffle plates are arranged to reach into the spaces intermediate the wing or worm screw flights for cooperation with these flights to convey the material to be leached in an upward direction against the leaching liquid flowing downwardly through the tower.
Several different modifications of extraction towers of the above type have become known whereby, for the purpose of avoiding jamming and the destruction of the sugar beets cossettes, the tower has been subdivided into a lower stirring zone and into an upper extraction zone adjacent to the stirring zone. The two zones are directly connected with each other. However, different type conveying elements are employed in the stirring zone and in the extraction zone whereby such different conveying elements are either secured to the central shaft or to the inner wall of the extraction tower, please see for example German Patent 1,017,549.
In spite of employing different conveying elements in different zones of an extraction tower, which might even include in addition to the stirring and extraction zone a discharge zone, it has not quite been possible heretofore to avoid local compacting or damming of the material, especially of the cossettes. Such local compactions may, for example, be caused by the fact that the cossettes have irregular dimensions or by the fact that the cossettes have different consistencies, for example, due to influences occurring at the time of pretreatment of the cossettes or due to the fact that different types of sugar beets are being used.
In order to achieve a substantially faultless operation of the tower it is necessary that the extraction or leaching liquid may flow downwardly through the entire cross sectional area of the tower in a uniform distribution through the material to be leached which is conveyed upwardly in the tower whereby simultaneously a sufficient residence time of the material in the tower must be assured. Any non-uniform compaction throughout the body of material being conveyed impairs the result of the extraction and its efficiency. In many instances compaction causes the destruction of the cossettes so that the latter are completely mashed thereby increasing the mashed pulp proportion relative to the useful cossettes. Such increased proportion of pulp impairs the further treatment of the juice as well as of the cossettes.
The above mentioned use of different conveying means in the individual zones of the tower improves the uniformity of the throughput as well as the uniform distribution of the material over the cross section of the tower as compared to prior art towers employing the same conveying means throughout the length of the tower. However, the tower comprising several zones and several different types of conveying means is rather expensive in its construction and in addition even the additional expense does not sufficiently assure that local compaction of the material to be leached is avoided with certainty.